Applications of the Global Positioning System

Prof. Thomas Herring

Department of Earth, Atmosphere and Planetary Sciences

12.080 Seminar Fall 2004.

http://geoweb.mit.edu/~tah

tah@mit.edu

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Overview

• Briefly review history of GPS: original aims of few-meter positioning

• Examine some MIT projects where GPS is used to make sub-millimeter position measurements and study deformation processes.– New Zealand– Oil field deformation– North America Plate deformation

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GPS Original Design (circa 1970)

• Started development in the late 1960s as NAVY/USAF project to replace Doppler positioning system

• Aim: Real-time positioning to < 10 meters, capable of being used on fast moving vehicles.

• Limit civilian (“non-authorized”) users to 100 meter positioning.

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GPS Design• Innovations:

– Use multiple satellites (originally 21, now ~28)– All satellites transmit at same frequency– Signals encoded with unique “bi-phase,

quadrature code” generated by pseudo-random sequence (designated by PRN, PR number): Spread-spectrum transmission.

– Dual frequency band transmission (allows propagation delay due to the ionosphere to be removed):• L1 ~1.5 GHz, L2 ~1.25 GHz

– Use of phase measurements allows millimeter level position determinations

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Latest Block IIR satellite(1,100 kg)

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• Total system cost is over $10B • Average cost per satellite is

~$60M• None of these costs are passed

directly to users.• Satellites transmit signals that

any one with the correct receivers can use (no use tax on receivers)

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Measurements• Measurements:

– Time difference between signal transmission from satellite and its arrival at ground station (called “pseudo-range”, precise to 0.1–10 m)

– Carrier phase difference between transmitter and receiver (precise to a few millimeters)

– Doppler shift of received signal• All measurements relative to “clocks” in ground

receiver and satellites (but use of multiple satellites and receivers allow this problem to be removed).

• High precision, dual frequency receivers now $4000-$6000

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Satellite constellation

• Since multiple satellites need to be seen at same time (four or more):– Many satellites (original 21 but now 28)– High altitude so that large portion of Earth

can be seen (20,000 km altitude —MEO)

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Current constellation

• Relative sizes correct (inertial space view)

• “Fuzzy” lines not due to orbit perturbations, but due to satellites being in 6-planes at 55o inclination.

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Some MIT projects using GPS

• The MIT Geodesy and Geodynamics group (http://geoweb.mit.edu) is involved in many projects around the world with using GPS.

• Three projects to discuss (all involving measurement of height changes)– Uplift in New Zealand Southern Alps– Subsidence in oil fields– Uplift and subsidence across North America (response to

last ice-age: Glacial Isostatic Adjustment GIA)• All these projects involve collaborations with other groups.

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A Direct Geodetic Measurement of the Uplift Rate of the Southern Alps

John Beavan1

Mikael Denham2

Paul Denys2

Brad Hager3

Tom Herring3

Chuck Kurnik4

Dion Matheson1

Peter Molnar5

Chris Pearson2

1 GNS2 Otago University3 MIT4 UNAVCO5 Univ. Colorado

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New ZealandTectonic and bathymetric

setting

Image from NIWANational Institute of Water and Atmospheric Research Ltd

Central South Island experiences oblique continental collision at about 40 mm/yr

Shortening component normal to Alpine fault is about 10 mm/yr

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SAGENZ Profile: Karangarua to Lake Tekapo

MTJO

QUAR

Christchurch

Southern Alps Geodetic Experiment - New Zealand

Pacific Plate

Australian Plate

Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center

40 mm

/yr

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OCCUPATION STRATEGY for CONTINUOUS and SEMI-CONTINUOUS GPS STATIONS

• Permanent ground marks at all sites, with force-centered antenna mounts

• Five sites occupied continuously, with data transmitted by radio modem and dial-up internet

• Six other sites occupied semi-continuously by rotating 3 receivers between the sites every 3 months

• 60-second sampling used to enable 3 months of data to be stored in internal memory

• Same antennas used at each site during each occupation

10/01/2004 12.080 GPS 14A Southern Alps semi-continuous GPS station

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Sometimes we have to find the sites

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And test snow integrity

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MTJO

CNCL

KARA

QUARNETT

MAKA

VEXA

WAKA

LEOC

PILK

REDD

Continuous station

Semi-continuous

SAGENZ GPS STATIONSA

lpin

e

Fau

ltNW SE

Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center

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Vertical position time series after

filtering to remove annual signals and

common-mode noise

• RMS about a straight line fit is typically 2.5 - 4 mm. Worst case (NETT) is 6.5 mm.

10/01/2004 12.080 GPS 19Measured uplift rates and 95% confidence

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For a complete change in climate:Oil field monitoring in Oman.Summer daytime temperatures of 50C (120F)

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Basic GPS setupSite at the suspected center of the subsidence

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Continuous GPS site (5 of these)

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GPS Monument and antenna

Passive cooled equipment box Telemetry antenna

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Drilled-braced monument

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Rover GPS sites (45 of these)

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355 mm

Pillar is ~3 meters deep

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Time series of height estimates in the center of the field

Receiver failure and repair

Error bars scaled based on “global” GPS analysis

Two analysesshown that treatatmospheric delaydifferently

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Horizontalmotion at REM3

Note: if the sitewere truly at the center of the subsidence, no horizontal motion would be expected

Some non-steady motion can be seen

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Deformation from 9-months GPS

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Postglacial rebound in North America

• Here is combine temporal changes in position estimates from ~300 sites across North America.

• These results are generated by many different groups and need to be carefully evaluated.

• The results from the best sites are used to infer which GIA models best match the measurements. (Depends on structure of the Earth)

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Fit of COD to GIA: 9/1995-3/2004: 17 sites

This minimummoves with increasing LT

Lithosphere Thickness (LT) 71 kmDetails here depend geographic sites distribution

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Comparison of PUR solution (red, 50% confidence ellipses) with GIA model 71 km LT, UM 1, LM 2x1021 Pa-s

Fit: 26-sitesN 0.6 mm/yrE 0.3 mm/yrU 1.9 mm/yr

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Time series of Site GDAC for North and Height

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Conclusions

• GPS dual-use technology: Applications in civilian world widespread– Geophysical studies (mm accuracy)– Engineering positioning (<cm in real-time)– Commercial positioning: cars, aircraft, boats (cm

to m level in real-time)• MIT has projects using GPS in many parts of the

world studying a variety of problems• Plate Boundary Observatory will be a major project

over the next 10 years in the US